Effects of Altering the Ratio of Indispensable to Dispensable Amino Acids in Diets for Rats

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Abstract
Male weanling rats were fed a series of amino acid diets in which both the nitrogen content and the ratio of nitrogen from indispensable and dispensable amino acids were varied. Growth, food consumption, and final body composition were determined for all experimental groups. The nitrogen of several of the amino acid diets was used very efficiently, indicating that the indispensable amino acids were present in well-balanced proportions. The amino acid diets which contained indispensable amino acids only or relatively large excesses of dispensable amino acids were inferior to diets with moderate amounts of both indispensable and dispensable amino acids. Rats were shown to be much less sensitive to the ratio of indispensable to dispensable amino acid nitrogen (I/D ratio) than are chicks. Growth of rats fed diets having I/D ratios between 4.0 and 1.0, was generally satisfactory, whereas earlier work had shown that chicks grew well only when the I/D ratio was near 2.0. On the basis of growth performance alone a diet with 1.8% of total nitrogen proved superior to the other amino acid diets. This diet contained equal amounts of nitrogen from both the dispensable and the indispensable amino acids. The performance of a group fed this diet and that of a group pair-fed a casein control diet were similar in all respects except protein efficiency ratio. When nitrogen utilization was used as the criterion of performance, diets containing less than 1.8% of total nitrogen were superior to diets with 1.8% of nitrogen or more. The term, “nitrogen incorporation efficiency,” was introduced to permit direct comparisons of efficiency of utilization of the nitrogen in these diets. This term represents the actual percentage of ingested nitrogen incorporated into the carcass and is based on body weight, food consumption and carcass nitrogen analysis.